There are a few ways a seller can set up postage discounts. Here's a simple way to tell if the discount applies to multiple different items, or just the listing you're looking at.
If, on the normal listing page, there is a 'See Discounts' link, the seller has set up postage rules that apply to more than one listing and will be automatically calculated at checkout if purchases are eligible for that discount. As there is more than one rule a seller can have, each listing must specify the same discount to qualify.
If that doesn't make sense, here's an example.
You click on the 'See Discounts' link and the pop-up says something like "50 cents postage per item when you purchase additional eligible items from [seller]". You look at another listing and click the link, but this one says "Free postage for purchases of $100 or more eligible items from [seller]". These are two different rules and the second one doesn't qualify for only 50c extra postage, nor does the first one qualify for free postage if you purchased $100 worth.
If there is no 'See Discounts' link, but when you look at the Postage and Payments tab there is $x.xx for each additional item, that discount applies to that listing only.
There's just one thing to remember - in variant listings, if you purchase one variation then go on to purchase a different variation and it only has the "$X.XX per additional item" in the P&P tab (so, not the see discounts bit) then the eBay checkout treats each purchase as if it was made from a separate listing and full postage is charged for each item unless the seller sends a revised invoice.
